The present invention relates to a method of immobilizing or killing swimming larvae in a mass of fresh water, and to an electric trap for practicing such method. In the method comprising use of the trap, an alternating electric field is generated between electrodes, at least one of which is insulated, in fresh water in order to influence larvae or similar living organisms (macro-organisms).
In this manner it is possible to deactivate or kill larvae without any use of chemistry (poison), in a manner which is favorable to the environment and capable of dealing with large water volumes. Such a system is important in connection with, inter alia, water inlets which are exposed to fouling by fresh-water organisms like fresh-water mussels.
Usually these problems are combated by the use of chemical methods. This means that a poison is introduced at an initial position in the system (e.g. a water inlet). This poison may have the form of a liquid (chlorine), a gas (ozone) or the shape of solid particles which float in the water and dissolve successively. Most of these chemical systems are of such a nature that all of or parts of the amount of poison return to the reservoir supplying the water. Thus the poison concentration may increase gradually, and this is not environmentally acceptable. In order to compensate for this effect, systems have been introduced where the poison is neutralized (e.g. by using clay), or unstable poisons (ozone) have been used, neutralizing themselves in a short time.
However, it is favorable to be able to avoid such environmental influences completely, and it has turned out to be possible to use electric effects in order to influence swimming organisms. However, it is not possible to find much literature regarding prior art in this field. Some studies have been made relating to how certain organisms behave in a DC voltage field (e.g. adjacent to a direct current conductor), and the influence from magnetic fields has also been studied to a certain degree. Relatively few studies have been made regarding the influence of electric AC fields on larvae and other macro-organisms.
Some experiments conducted in Russia by M. Y. Kirpichenko show that relatively weak electric fields (7-10 volts/cm), exposing larvae during a long period (30-50 hours), depending on water temperature, reduce the larvae deposit and provides a high mortality (above 90%).
There have also in Russia been executed extensive experiments leading to certain pilot installations in power plants, where the water passes through an electrostatic filter with 50 Hz sinusoidal AC voltages on the order of 220-450 volts, depending on the stage of development of the larvae. In general it is possible to state that the larvae must be exposed at least one second for mortality, however this time can be reduced by altering the shape of the pulses.
One system for preventing fouling which is based upon the use of an electric field in connection with electrolysis is also known, such a system being published in a patent application assigned to Mitsubishi, Japan, corresponding to EP 0 369 557, published May 23, 1990. The problem of such an electrolysis system is, however, that the electrodes are in direct contact with the water, and hence there flows a strong electric current which mainly heats the water. Therefore, such a system is usually not very cost-efficient.
Most experiments have been conducted with larvae in sea water, and these experiments show that an electric field does not affect the larvae directly, inter alia, because the current "flows" around and past the larva.
From Norwegian Patent No. 83894, Swedish Patent No. 140958 and German Offenlegungsschrift No. 1073629 there are known fish catch means for use in salt water, where a pulsed current is used, i.e. a DC current with superposed AC components. Thereby the fish is intended to be stunned or killed by electric current conduction. Very high voltages are used, and since salt water is a good electric conductor, large currents will flow, i.e. the generator supplying current to the electrodes in the water must be able to deliver a very high electric power.
It is desirable to be able to immobilize or kill larvae also in fresh water, and preferably using an alternating electric field which can be generated without power dissipation of the same magnitude or order as is the case in salt water.